1. Field of the Invention
The present invention relates to an actuator having, for example, a mirror mounted to an end of a movable shaft. More particularly, the present invention relates to an actuator that can prevent a rotational fulcrum of a movable shaft from becoming displaced from a tilt center point when the movable shaft is tilted, and that can prevent rotation around the movable shaft.
2. Description of the Related Art
Recording of two-dimensional digital signals onto a holographic recording medium by multiplexing or reproducing of the two-dimensional digital signals from the holographic recording medium by multiplexing is generally performed by changing a wavelength or an angle of incidence of reference light incident upon the recording medium. A typical example of a related actuator that adjusts the angle of incidence of a light beam (reference light) is a galvanometer mirror (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-271821 (page 6, FIGS. 8 and 9).
Japanese Unexamined Patent Application Publication No. 2004-271821 discusses a biaxial planar galvanometer mirror formed using a semiconductor manufacturing technology. In the galvanometer mirror, an outer frame is supported by a supporting member using a first torsion bar (which forms one shaft), and an inner frame (provided with a mirror) is supported by the outer frame using a second torsion bar (which forms another shaft). When an electromagnetic force is generated at a magnetic driving unit, twisting occurs with the first torsion bar and the second torsion bar serving as centers and disposed orthogonal to each other. Therefore, the outer frame and the inner frame are inclined like seesaws, thereby making it possible to freely change the angle of the mirror.
The supporting means of the galvanometer mirror discussed in Japanese Unexamined Patent Application Publication No. 2004-271821 is what is called a gimbal mechanism, in which the inner frame and the outer frame are independently rotated by the twisting of the first and second torsion bars. Therefore, a support center point of the mirror (control object that is controlled) matches the center of the first torsion bar and the center of the second torsion bar.
However, the support center point with which the two centers match greatly depends upon the precision of individual gimbal mechanisms. Moreover, since the structure makes use of twisting, the support center point varies with each gimbal mechanism, thereby making it difficult to set the center points on the mechanisms at one point.
For example, when reproduction is performed on a holographic recording medium, the hologram recording medium is scanned by swinging reference light while adjusting the mirror in accordance with angles. Here, the support center point corresponds to the center of angle scanning. However, if, as mentioned above, the support center point is not determined, a spot of the reference light is shifted from a predetermined location, thereby generating a read-out error during the scanning.
What is called an Earth-top-type actuator is discussed in Japanese Patent Application No. 2005-259060 previously filed by the applicant and not presented as a prior art document because it has not yet been laid open when filling the application. In this actuator, a movable shaft provided with a mirror is rotatably mounted to a movable ring through a pair of small balls (first small balls). The movable ring is rotatably mounted to a securing base through a pair of small balls (second small balls). The first small balls and the second small balls form two axes that are orthogonal to each other. The mirror can swing around the two axes.
However, although the actuator discussed in Japanese Patent Application No. 2005-259060 is mechanically excellent, it is expensive. Therefore, there is a demand for developing an actuator that is low in cost and that can emulate the function of the actuator discussed in Japanese Patent Application No. 2005-259060.